It is known that, synthetic fibers are conventionally made by extruding through a die a molten polymeric mass. This die essentially comprises a plate provided with a plurality of very small holes, at the outlets of which a corresponding number of very thin fibers are formed. By this apparatus it is possible to also make the so-called "two-component fibers", that is yarns formed from a combination of two different polymers. In this case, the molten polymeric masses are separately supplied to the extruding die, at the outlet of which are obtained composite fibers made of the two polymers (for example the so-called "side-by-side" or "sheath-core" fibers).
A critical aspect of these prior methods for making the above mentioned fibers, is constituted by the molten polymeric mass supply to the extruding die. In fact, the viscosity of the materials to be extruded, together with the very complex configuration and very small size of the channels provided for distributing the mentioned materials, will involve modifications of the design parameters related to the supply of the molten polymeric masses to the extruding die. In particular, great differences are encountered through the polymeric mass supplied to the die, with respect to the pressure, rate, temperature and viscosity values of said mass, which differences will cause in turn unevenesses in the supply of the polymers to the extruding apparatus. Because of the mentioned reasons, the amounts of extruded materials are not constant and the yarn material exiting the die has a randomly carrying count (the diameters of the fibers being very different). In particular, in making two-component fibers, the yarn is conventionally richer in the polymeric material of lower density and having a lower viscosity, and, generally, the obtained fiber includes therein the two polymers in a randomly varying ratio.